feat(i18n): add Swahili locale selector (#669)

[nupedev]

Summary

Closes #669

Expands the frontend language experience to English, French, and Swahili while preserving the existing next-intl and localStorage approach.

Changes

• Adds a complete Swahili catalog covering the existing English message keys, including dashboard labels, errors, payment plans, help and accessibility text, reporting, history, and tooltips.
• Retains and exposes the existing English and French catalogs through a shared locale option list.
• Replaces the binary Navbar toggle with responsive desktop and mobile selectors labeled English, Français, and Kiswahili.
• Persists the selected locale under sg_locale, restores supported values on load, and cycles through all three locales through the provider API.
• Adds focused provider tests for Swahili restoration, persistence, explicit selection, and three-locale cycling.

Verification

• npm test -- --runInBand src/__tests__/I18nProvider.test.tsx — 2 passed.
• git diff --check — passed.
• Full TypeScript checking continues to report unrelated pre-existing Jest global/type-definition errors in the repository test suite; the changed provider, Navbar, and Swahili catalog have no TypeScript diagnostics.

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